50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

in the TVZ and southern Havre Trough; 12 and 15
km in the western and eastern Bay of Plenty
shelves, respectively). For each earthquake source,
we derive earthquake moment magnitudes (Mw)
using an empirical regression relating rupture
dimensions to Mw. We assumed Mw 5.8 as a lower
threshold for ground rupturing earthquakes in the
TVZ, corresponding to a rupture length of 6-7 km,
according to published length-magnitude empirical
correlations from a global compilation of faults. To
estimate slip rates, we mainly use (1) the
displacement of the 7 to 20 kyr old post-last glacial
transgressive surface (PGS) in water depths
shallower than 150 m; and (2) the estimated
contribution of individual faults to the total tectonic
extension (12-20 mm/yr) across the Bay of Plenty
continental slope and deep water basins, on the
basis of fault scarp geomorphology.
We interpreted 166 individual potential earthquake
sources in the offshore Bay of Plenty, and 36
composite scenarios by considering the combined
rupture of adjacent offshore sources during an
earthquake, or the extension of an offshore rupture
onto land. Most of the faults are previously
unrecognised in seismic hazard models, in
particular the reverse faults in the eastern Bay of
Plenty, and normal faults close to Tauranga.
Assigned rupture lengths range from 6 to 51 km,
and are associated with magnitudes ranging from
Mw 5.7 to 7.1. More than 75% of the earthquake
sources could generate Mw 6.0 to 6.6. From
estimates of the seismic moment (Mo) and slip rate,
we determine single event displacements and
earthquake recurrence intervals. Estimates of
coseismic displacements averaged across the fault
surfaces range from as little as 0.1 m to 2.4 m, and
are likely to be exceeded locally at the ground
surface. Recurrence intervals range from 20 to
58,000 years, with an average of ~3000 yrs. There
are 25 earthquake sources that have minimum
recurrence intervals of less than 100 years, whereas
intervals for the sources that probably extend
onshore are more than 1000 yrs.
ORAL
USING TREE COLONISATION TO
INVESTIGATE THE AGE OF THE MOST
RECENT EARTHQUAKE ALONG THE
WESTERN HOPE FAULT
R. M. Langridge 1 ,R.Duncan 2 ,V.Perez 3 ,M.
Smith 2 &P.Almond 2
1 Institute of Geological & Nuclear Sciences, P.O
Box 30-368, Lower Hutt, NZ
2 Division of Bioprotection and Ecology, Lincoln
University, Christchurch, NZ
3 Department of Geology, Granada, Spain
(r.langridge*gns.cri.nz)
The Hope Fault is the highest slip rate fault in
northern South Island and constitutes the major
neotectonic link between the Alpine Fault and
Hikurangi Subduction Zone - the main plate
boundary structures through NZ. The slip rate in the
western part of the Hope Fault (Hurunui section) is
c. 12 mm/yr and the recurrence time for surface
faulting events is c. 310-490 yr. Though the Alpine
Fault has an established pre-European fault rupture
event (c. 1717 AD) and the Hope River segment of
the Hope Fault ruptured in 1888 AD, there is no
known rupture record for the Hurunui section.
We have cored 265 beech trees at 15 sites on or
adjacent to the active trace of the Hope Fault in
order to test whether massive landscape changes
during the last 300 yr have been recorded in the
growth history of Beech forests there. About 65%
of the trees sampled were Red (Nothofagus fusca);
28% Silver (N. menziesii); and 7% Mountain (N.
solandri var cliffortiodes) Beech. We compare our
preliminary results with 14-C dates from sites of
landscape change, tree disturbance and one handdug
trench along the Hurunui section to determine a
“most-recent” fault rupture date for the western end
of the Hope Fault. Our geomorphic model for large
seismic events is for: i) c. 3.4 m average dextral
displacement on the fault; ii) high levels of ground
shaking capable of affecting patches of trees along
the fault trace; (iii) landsliding and sediment
generation from smaller catchments surrounding
the Hurunui valley; and (iv) deposition of fluvial
and fan deposits after the earthquake.
A preliminary analysis of the data suggest there are
three major forest re-generation events that can be
picked out from multiple sites. They correspond in
time to the AD 1888 event; an event in the earlymid
19 th century, and an event in the early-mid 18 th
century.
ORAL
TEREBRATULIDE AND
RHYNCHONELLIDE BRACHIOPODS IN
NEW ZEALAND – WINNERS AND LOSERS
OF THE MESOZOIC MARINE
REVOLUTION
D. E. Lee 1 &D.A.B.MacFarlan 2
1 Department of Geology, University of Otago, PO
Box 56, Dunedin
2 MacFarlan Geological Services Ltd, 13 Fairfax
Terrace, New Plymouth
(daphne.lee*stonebow.otago.ac.nz)
The end-Permian extinction event removed many
of the brachiopod clades that had dominated
Paleozoic sea floors, and decimated the remainder.
By the end of the Early Jurassic, spiriferides and
athyrides, members of two of the five articulated
50 th Kaikoura05 -43- Kaikoura 2005